An ATI Radeon 5870 will be a great boost. You can also get flashed OC cards with 1GB memory. When using Mac Pro 2.1 had an ATI Radeon HD4870 1GB card and it was greater. Either way consider updating to Snow Leopard OS X.6 at least, or Lion OS X.7.

Hang on to those original install discs like grim death! Using OS X.7 or later make a bootable USB thumb drive before running Installer!